Chordate
fossils have been found from as early as the
Cambrian explosion, 539 million years ago.[7]Cladistically (
phylogenetically),
vertebrates – chordates with the notochord replaced by a
vertebral column during development – are a subgroup of the
cladeCraniata, which consists of chordates with a
skull. Of the more than 81,000[8] living species of chordates, about half are ray-finned fishes that are members of the class
Actinopterygii and the vast majority of the rest are
tetrapods (mostly birds and mammals).
History of name
Although the name Chordata is attributed to
William Bateson (1885), it was already in prevalent use by 1880.
Ernst Haeckel described a taxon comprising tunicates, cephalochordates, and vertebrates in 1866. Though he used the German vernacular form, it is allowed under the
ICZN code because of its subsequent latinization.[4]
Anatomy
Chordates form a
phylum of animals that are defined by having at some stage in their lives all of the following anatomical features:[9]
Pharyngeal slits. The
pharynx is the part of the
throat immediately behind the
mouth. In
fish, the slits are modified to form
gills, but in some other chordates they are part of a
filter-feeding system that extracts food particles from ingested water. In
tetrapods, they are only present during embryonic stages of the development.
A post-anal tail. A muscular tail that extends backwards behind the
anus. In some chordates such as humans, this is only present in the embryonic stage.
An
endostyle. This is a groove in the
ventral wall of the pharynx. In
filter-feeding species it produces
mucus to gather food particles, which helps in transporting food to the
esophagus.[10] It also stores
iodine, and may be a precursor of the vertebrate
thyroid gland.[9]
There are soft constraints that separate chordates from other biological lineages, but are not part of the formal definition:
All chordates are
deuterostomes. This means that, during the embryo development stage, the anus forms before the mouth.
Anatomy of the
cephalochordateAmphioxus. Bolded items are components of all chordates at some point in their lifetimes, and distinguish them from other phyla.
Superclass Tetrapoda (four-limbed vertebrates; 35,100+ species) (The classification below follows Benton 2004, and uses a synthesis of rank-based Linnaean taxonomy and also reflects evolutionary relationships. Benton included the Superclass Tetrapoda in the Subclass Sarcopterygii in order to reflect the direct descent of tetrapods from lobe-finned fish, despite the former being assigned a higher taxonomic rank.)[16]
Cephalochordates, one of the three subdivisions of chordates, are small, "vaguely fish-shaped" animals that lack brains, clearly defined heads and specialized sense organs.[21] These burrowing filter-feeders compose the earliest-branching chordate subphylum.[22][23]
Most
tunicates appear as adults in one of two major forms, known as "sea squirts" and
salps. Both of these are soft-bodied filter-feeders that lack the standard features of chordates, which are only retained in their larvae. Sea squirts are sessile and consist mainly of water pumps and filter-feeding apparatus;[24]salps float in mid-water, feeding on
plankton, and have a two-generation cycle in which one generation is solitary and the next forms chain-like
colonies.[25] However, all tunicate
larvae have the standard chordate features, including long,
tadpole-like tails; they also have rudimentary brains, light sensors and tilt sensors.[24] The third main group of tunicates,
Appendicularia (also known as Larvacea), retain tadpole-like shapes and active swimming all their lives, and were for a long time regarded as larvae of sea squirts or salps.[26] The etymology of the term Urochordata (Balfour 1881) is from the ancient Greek οὐρά (oura, "tail") + Latin chorda ("cord"), because the notochord is only found in the tail.[27] The term Tunicata (Lamarck 1816) is recognised as having precedence and is now more commonly used.[24]
Most craniates are
vertebrates, in which the
notochord is replaced by the
vertebral column.[29] It consists of a series of bony or cartilaginous
cylindrical vertebrae, generally with
neural arches that protect the
spinal cord, and with projections that link the vertebrae.
Hagfishes have incomplete
braincases and no vertebrae, and are therefore not regarded as vertebrates,[30]
but they are members of the craniates, the group within which vertebrates are thought to have
evolved.[31] However the cladistic exclusion of hagfish from the vertebrates is controversial, as they may instead be degenerate vertebrates who have secondarily lost their vertebral columns.[32]
The position of
lampreys is ambiguous. They have complete braincases and rudimentary vertebrae, and therefore may be regarded as vertebrates and true
fish.[33] However,
molecular phylogenetics, which uses
biochemical features to classify organisms, has produced both results that group them with vertebrates and others that group them with hagfish.[34] If lampreys are more closely related to the hagfish than the other vertebrates, this would suggest that they form a
clade, which has been named the
Cyclostomata.[35]
Phylogeny
Overview
There is still much ongoing differential (DNA sequence based) comparison research that is trying to separate out the simplest forms of chordates. As some lineages of the 90% of species that lack a backbone or notochord might have lost these structures over time, this complicates the classification of chordates. Some chordate lineages may only be found by DNA analysis, when there is no physical trace of any chordate-like structures.[37]
Attempts to work out the evolutionary relationships of the chordates have produced several hypotheses. The current consensus is that chordates are
monophyletic, meaning that the Chordata include all and only the descendants of a single common ancestor, which is itself a chordate, and that
craniates' nearest relatives are tunicates. Recent identification of two
conserved signature indels (CSIs) in the proteins cyclophilin-like protein and mitochondrial inner membrane protease ATP23, which are exclusively shared by all
vertebrates,
tunicates and
cephalochordates also provide strong evidence of the monophyly of Chordata.[5]
All of the earliest chordate
fossils have been found in the Early
CambrianChengjiang fauna, and include two species that are regarded as
fish, which implies that they are vertebrates. Because the fossil record of early chordates is poor, only
molecular phylogenetics offers a reasonable prospect of dating their emergence. However, the use of molecular phylogenetics for dating evolutionary transitions is controversial.
It has also proved difficult to produce a detailed classification within the living chordates. Attempts to produce evolutionary "
family trees" shows that many of the traditional
classes are
paraphyletic.
While this has been well known since the 19th century, an insistence on only monophyletic taxa has resulted in vertebrate classification being in a state of flux.[38]
The majority of animals more complex than
jellyfish and other
Cnidarians are split into two groups, the
protostomes and
deuterostomes, the latter of which contains chordates.[39] It seems very likely the 555 million-year-oldKimberella was a member of the protostomes.[40][41] If so, this means the protostome and deuterostome lineages must have split some time before Kimberella appeared—at least 558 million years ago, and hence well before the start of the Cambrian 538.8 million years ago.[39] The
Ediacaran fossil Ernietta, from about 549 to 543 million years ago, may represent a deuterostome animal.[42]
Fossils of one major deuterostome group, the
echinoderms (whose modern members include
starfish,
sea urchins and
crinoids), are quite common from the start of the Cambrian, 542 million years ago.[43] The Mid
Cambrian fossil Rhabdotubus johanssoni has been interpreted as a
pterobranch hemichordate.[44] Opinions differ about whether the
Chengjiang fauna fossil Yunnanozoon, from the earlier Cambrian, was a hemichordate or chordate.[45][46] Another fossil, Haikouella lanceolata, also from the Chengjiang fauna, is interpreted as a chordate and possibly a craniate, as it shows signs of a heart, arteries, gill filaments, a tail, a neural chord with a brain at the front end, and possibly eyes—although it also had short tentacles round its mouth.[46]Haikouichthys and Myllokunmingia, also from the Chengjiang fauna, are regarded as
fish.[36][47]Pikaia, discovered much earlier (1911) but from the Mid Cambrian
Burgess Shale (505 Ma), is also regarded as a primitive chordate.[48] On the other hand, fossils of early chordates are very rare, since invertebrate chordates have no bones or teeth, and only one has been reported for the rest of the Cambrian.[49]
The evolutionary relationships between the chordate groups and between chordates as a whole and their closest deuterostome relatives have been debated since 1890. Studies based on anatomical,
embryological, and paleontological data have produced different "family trees". Some closely linked chordates and hemichordates, but that idea is now rejected.[10] Combining such analyses with data from a small set of
ribosomeRNA genes eliminated some older ideas, but opened up the possibility that tunicates (urochordates) are "basal deuterostomes", surviving members of the group from which echinoderms, hemichordates and chordates evolved.[50] Some researchers believe that, within the chordates, craniates are most closely related to cephalochordates, but there are also reasons for regarding tunicates (urochordates) as craniates' closest relatives.[10][51]
Since early chordates have left a poor fossil record, attempts have been made to calculate the key dates in their evolution by
molecular phylogenetics techniques—by analyzing biochemical differences, mainly in RNA. One such study suggested that deuterostomes arose before 900 million years ago and the earliest chordates around 896 million years ago.[51] However, molecular estimates of dates often disagree with each other and with the fossil record,[51] and their assumption that the
molecular clock runs at a known constant rate has been challenged.[52][53]
Traditionally, Cephalochordata and Craniata were grouped into the proposed clade "Euchordata", which would have been the sister group to Tunicata/Urochordata. More recently, Cephalochordata has been thought of as a sister group to the "Olfactores", which includes the craniates and tunicates. The matter is not yet settled.
A specific relationship between Vertebrates and
Tunicates is also strongly supported by two CSIs found in the proteins predicted exosome complex RRP44 and serine palmitoyltransferase, that are exclusively shared by species from these two subphyla but not
Cephalochordates, indicating Vertebrates are more closely related to
Tunicates than
Cephalochordates.[5]
Cladogram
Below is a
phylogenetic tree of the phylum. Lines of the
cladogram show probable evolutionary relationships between both
extinct taxa, which are denoted with a
dagger (†), and
extant taxa. Relatives of vertebrates are
invertebrates. The positions (relationships) of the lancelets, tunicates, and craniates/vertebrates are based on the following studies:[54][55][56][57]
The closest relatives of the Chordates are believed to be the
Hemichordates and
Echinodermata, which together form the
Ambulacraria.
The Chordata and Ambulacraria together form the superphylum
Deuterostomia.
Hemichordates ("half chordates") have some features similar to those of chordates: branchial openings that open into the
pharynx and look rather like gill slits; stomochords, similar in composition to
notochords, but running in a circle round the "collar", which is ahead of the mouth; and a
dorsal nerve cord—but also a smaller
ventral nerve cord.
There are two living groups of hemichordates. The solitary
enteropneusts, commonly known as "acorn worms", have long
proboscises and worm-like bodies with up to 200 branchial slits, are up to 2.5 metres (8.2 ft) long, and burrow though
seafloor sediments.
Pterobranchs are
colonial animals, often less than 1 millimetre (0.039 in) long individually, whose dwellings are interconnected. Each
filter feeds by means of a pair of branched tentacles, and has a short, shield-shaped proboscis. The extinct
graptolites, colonial animals whose fossils look like tiny
hacksaw blades, lived in tubes similar to those of pterobranchs.[58]
Echinoderms differ from chordates and their other relatives in three conspicuous ways: they possess
bilateral symmetry only as larvae – in adulthood they have
radial symmetry, meaning that their body pattern is shaped like a wheel; they have
tube feet; and their bodies are supported by
skeletons made of
calcite, a material not used by chordates. Their hard, calcified shells keep their bodies well protected from the environment, and these skeletons enclose their bodies, but are also covered by thin skins. The feet are powered by another unique feature of echinoderms, a
water vascular system of canals that also functions as a "lung" and surrounded by muscles that act as pumps.
Crinoids look rather like flowers, and use their feather-like arms to filter food particles out of the water; most live anchored to rocks, but a few can move very slowly. Other echinoderms are mobile and take a variety of body shapes, for example
starfish,
sea urchins and
sea cucumbers.[59]
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